1. Field of the Invention
This invention relates to a cam phase control system for an internal combustion engine, for controlling a cam phase, as a phase of at least one of an intake cam and an exhaust cam relative to a crankshaft of the engine.
2. Description of the Related Art
As the cam phase control system of this kind, the present assignee has already proposed one disclosed in Japanese Patent Application No. 2004-70312. At one end of an intake camshaft of the engine, there is provided a hydraulic variable cam phase mechanism. The variable cam phase mechanism is comprised of a housing integrally formed with a sprocket, vanes received in the housing, an advancing hydraulic chamber and a retarding hydraulic chamber formed between the housing and the vane, and an electromagnetic control valve that changes hydraulic pressures supplied to the chambers. Further, the sprocket is connected to a crankshaft via a timing belt, and the vanes are connected to the intake camshaft for rotation in unison therewith.
In the variable cam phase mechanism constructed as above, the hydraulic pressures supplied to the advancing hydraulic chamber and the retarding hydraulic chamber are varied by the electromagnetic control valve to thereby steplessly vary the phase between the sprocket and the intake camshaft, i.e. the phase (hereinafter referred to as “the cam phase”) Cain of the intake cam relative to the crankshaft. Further, the electromagnetic control valve closes oil passages to thereby maintain the hydraulic pressures in the advancing and retarding hydraulic chambers, whereby the cam phase Cain is held at a value assumed then.
Further, the cam phase control system controls the cam phase Cain via the variable cam phase mechanism such that the cam phase Cain converges to a target cam phase Cain_cmd, and includes a crank angle sensor, a cam angle sensor, and an ECU. In the cam phase control system, the ECU controls the cam phase Cain in the following manner: The cam phase Cain is calculated based on detections signals from the crank angle sensor and the cam angle sensor, and the target cam phase Cain_cmd is calculated based on engine speed NE and an accelerator opening AP.
Further, with a target value filter-type two-degree-of-freedom control algorithm, a control value Rcain is calculated for causing the cam phase Cain to converge to the target cam phase Cain_cmd, and a control input Ucain to the variable cam phase mechanism is calculated by modulating the control value Rcain with a modulation algorithm based on a ΔΣ modulation algorithm. In the modulation algorithm, the control input Ucain has a modulation range thereof set to a predetermined value, and a value Ucain_off_adp as the center of the modulation range is set according to the value of the control value Rcain.
By calculating the control input Ucain as described above, even when the control value Rcain is varied or fluctuated due to heat generated by the solenoid of the control valve, variation in operating characteristics of individual variable cam phase mechanisms, and aging, it is possible to calculate the control input Ucain as such a value as will cause frequent occurrence of a switching behavior peculiar to the ΔΣ modulation algorithm, while limiting the modulation range of the control input Ucain to a relatively small value. As a result, it is possible to obtain high control accuracy in the cam phase control.
Further, as the variable cam phase mechanism, the present assignee has already proposed one described in Japanese Patent Application No. 2003-289910. The variable cam phase control mechanism, which is of an electromagnetic type, is disposed between a sprocket and an intake camshaft, and includes an electromagnet and a return spring. In the variable cam phase mechanism, the electromagnetic force of the electromagnet is varied against the urging force of the return spring, whereby the cam phase Cain is steplessly varied. Further, when the electromagnetic force is held constant, the cam phase Cain is held at a value in which the electromagnetic force and the urging force of the return spring are balanced.
The latter electromagnetic variable cam phase mechanism is advantageous over the hydraulic type in that dead time is shorter, and a higher responsiveness can be secured. Therefore, it is envisaged to apply the latter electromagnetic variable cam phase mechanism to the former cam phase control system. However, in this case, there can arise the following problems:
Since the variable cam phase mechanism is provided between the intake camshaft and the sprocket, and at the same time the sprocket is connected to the crankshaft via the timing belt (or timing chain), when a sudden accelerator pedal operation, a sudden brake pedal operation, or a clutch operation during a shift change causes a sudden increase in the engine speed, there can be caused a sudden change in the inertial force of the timing belt, the crankshaft etc. acting on the sprocket. In the electromagnetic variable cam phase mechanism, the cam phase Cain is varied by changing the relative magnitude relationship between the electromagnetic force and the urging force of the return spring, and hence if there occurs a sudden change in the inertial force, the magnitude relationship deviates from the proper relationship determined by the control input, which sometimes causes the cam phase Cain to deviate from a value corresponding to the control input. As a result, the convergence of the cam phase Cain to the target cam phase Cain_cmd is degraded, and as the control accuracy of the cam phase control lowers, the combustion state and the exhaust emission can be degraded.
As one method of solving the above problems, it is contemplated to set the modulation range of the control input Ucain to a relatively large range so as to accommodate a variation in the control value Rcain caused by deviation of the cam phase Cain. However, in this method, the size of the modulation range of the control input Ucain is reflected on the cam phase Cain itself, which can lower the controllability thereof on the contrary. The above problem also occurs when the ΔΣ modulation algorithm is replaced by a Δ modulation algorithm or ΣΔ modulation algorithm.